Architecture:
Mobile Station (MS):
Mobile
Equipment (ME)
Subscriber Identity
Module (SIM)
Base Station Subsystem (BSS):
Base
Transceiver Station (BTS)
Base Station
Controller (BSC)
Network Switching Subsystem (NSS):
Mobile
Switching Center (MSC)
Home Location
Register (HLR)
Visitor
Location Register (VLR)
Authentication
Center (AUC)
Equipment
Identity Register (EIR)
Mobile Equipment:
a. Portable,
vehicle mounted, hand held device.
b. Uniquely
identified by an IMEI (International Mobile Equipment Identity).
c. Voice
and data transmission.
d. Monitoring
power and signal quality of surrounding cells for optimum handover.
e. Power
level: 0.8W – 20 W.
f.
160 character long SMS.
Subscriber Identity Module (SIM):
a. Smart
card contains the International Mobile Subscriber Identity (IMSI).
b. Allows
user to send and receive calls and receive other subscribed services.
c. Encoded network identification details -
Key Ki, Kc and A3, A5 and A8 algorithms.
d. Protected by a password or PIN.
e. Can be moved from phone to phone – contains
key information to activate the phone.
Base Transceiver Station (BTS):
a. Encodes,
encrypts, multiplexes, modulates and feeds the RF signals to the antenna.
b. Frequency
hopping.
c. Communicates
with Mobile station and BSC.
d. Consists
of Transceivers (TRX) units.
Base Station Controller (BSC):
a. Manages
Radio resources for BTS.
b. Assigns
Frequency and time slots for all MS’s in its area.
c. Handles
call set up.
d. Trans
coding and rate adaptation functionality.
e. Handover
for each MS.
f.
Radio Power control.
g. It
communicates with MSC and BTS.
Mobile Switching Center (MSC):
a. Heart
of the network.
b. Manages
communication between GSM and other networks.
c. Call
setup function and basic switching.
d. Call
routing.
e. Billing
information and collection.
f. Mobility management
-
Registration
-
Location Updating
-
Inter BSS and inter MSC call handoff
g. MSC does gateway function while its customer
roams to other network by using HLR/VLR.
GMSC (Gateway MSC):
Handles all traffic to and from GSM and external
circuit switched networks, such as PSTN, ISDN or other mobile networks.
SGSN (Serving GPRS Support Node and GGSN):
Gateway GPRS Support Node has similar functionality
as MSC / GMSC, but for the packet switched part of the network. GGSN handles
connections to external IP networks.
Home Location Registers (HLR):
a. Permanent
database about mobile subscribers in a large service area(generally one per GSM
network operator).
b. Database
contains IMSI (International Mobile Subscriber Identity), MSISDN (Mobile Subscriber
Integrated Services Digital Network), roaming restrictions, and supplementary
services.
MSISDN (15 Digits):
Country Code [1-3]
|
National Mobile Number
|
IMSI (15 Digits):
Mobile County Code (MCC) [3]
|
Network Code (NC) [2-3]
|
Subscription Identification
|
Visitor Location Registers (VLR):
a. Temporary
database which updates whenever new MS enters its area, by HLR database.
b. Controls
those mobiles roaming in its area.
c. Reduces
number of queries to HLR.
Database
contains IMSI, TMSI (Temporary Mobile Subscriber Identity), MSISDN, MSRN
(Mobile Station Roaming Number), Location Area (LA), authentication key.
Authentication Center (AUC):
a. Protects
against intruders in air interface.
b. Maintains
authentication keys and algorithms and provides security triplets (RAND [Non
predictable random number], SRES [Signed response], Kc [Ciphering key]).
c. Generally
associated with HLR.
Equipment Identity Register (EIR):
a. Database
that is used to track handsets using the IMEI (International Mobile Equipment
Identity).
b. Made
up of three sub-classes: The White List (IMEI, assigned to valid ME), The Black
List (IMEI reported stolen) and the Gray List (IMEI having problems like faulty
software, wrong make of equipment etc).
c. Only
one EIR per PLMN (Public Land Mobile Network).
GPRS
support nodes (GSN):
A GSN is a network node which
supports the use of GPRS in the GSM core network. All GSNs should have a Gn
interface and support the GPRS tunneling protocol. There are two key variants
of the GSN, namely Gateway and Serving GPRS support node.
Gateway
GPRS support node (GGSN):
The gateway GPRS support node
(GGSN) is a main component of the GPRS network. The GGSN is responsible for the
internetworking between the GPRS network and external packet switched networks,
like the Internet and X.25 networks.
From an external network's
point of view, the GGSN is a router to a "sub-network", because the
GGSN ‘hides’ the GPRS infrastructure from the external network. When the GGSN
receives data addressed to a specific user, it checks if the user is active. If
it is, the GGSN forwards the data to the SGSN serving the mobile user, but if
the mobile user is inactive, the data is discarded. On the other hand,
mobile-originated packets are routed to the right network by the GGSN.
The GGSN is the anchor point
that enables the mobility of the user terminal in the GPRS/UMTS networks. In
essence, it carries out the role in GPRS equivalent to the home agent in Mobile
IP. It maintains routing necessary to tunnel the protocol data units (PDUs) to
the SGSN that services a particular MS (mobile station).
The GGSN converts the GPRS
packets coming from the SGSN into the appropriate packet data protocol (PDP)
format (e.g., IP or X.25) and sends them out on the corresponding packet data
network. In the other direction, PDP addresses of incoming data packets are
converted to the GSM address of the destination user. The readdressed packets
are sent to the responsible SGSN. For this purpose, the GGSN stores the current
SGSN address of the user and his or her profile in its location register. The
GGSN is responsible for IP address assignment and is the default router for the
connected user equipment (UE). The GGSN also performs authentication and
charging functions.
Other functions include
subscriber screening, IP pool management and address mapping, QoS and PDP
context enforcement.
With LTE scenario the GGSN
functionality moves to SAE gateway (with SGSN functionality working in MME).
Serving
GPRS support node (SGSN):
A serving GPRS support node
(SGSN) is responsible for the delivery of data packets from and to the mobile
stations within its geographical service area. Its tasks include packet routing
and transfer, mobility management (attach/detach and location management),
logical link management, and authentication and charging functions. The
location register of the SGSN stores location information (e.g., current cell,
current VLR) and user profiles (e.g., IMSI, address used in the packet data
network) of all GPRS users registered with it.
Common
SGSN functions:
a. Detunnels
GTP packets from the GGSN (downlink).
b. Tunnel
IP packets toward the GGSN (uplink).
c. Carry
out mobility management as Standby mode mobile moves from one Routing Area to
another Routing Area.
d. Billing
user data.
GSM/EDGE
specific SGSN functions:
Enhanced Data Rates for GSM
Evolution (EDGE) specific SGSN functions and characteristics are:
a. Maximum
data rate of approx. 60 kbit/s (150 kbit/s for EDGE) per subscriber.
b. Connect
via frame relay or IP to the packet control unit using the Gb protocol stack.
c. Accept
uplink data to form IP packets.
d. Encrypt
down-link data, decrypt up-link data.
e. Carry
out mobility management to the level of a cell for connected mode mobiles.
WCDMA specific SGSN functions:
a. Carry
up to about 42 Mbit/s traffic downlink and 5.8 Mbit/s traffic uplink
(HSPA+).
b. Tunnel/detunnel
downlink/uplink packets toward the radio network controller (RNC).
c. Carry
out mobility management to the level of an RNC for connected mode mobiles.
Access
point:
An access point is:
a. An
IP network to which a mobile set can be connected.
b. A
set of settings which are used for that connection.
c. A
particular option in a set of settings in a mobile phone.
d. When
a GPRS mobile phone sets up a PDP context, the access point is selected. At
this point an Access Point Name (APN) is determined.
Example:
aricenttechnologies.mnc012.mcc345.gprs, Internet, mywap, hcl.cisco.ggsn.
This access point is then used
in a DNS query to a private DNS network. This process (called APN resolution)
finally gives the IP address of the GGSN which should serve the access point.
At this point a PDP context can be activated.
PDP
context:
The packet data protocol (PDP;
e.g., IP, X.25, Frame Relay) context is a data structure present on both the serving
GPRS support node (SGSN) and the gateway GPRS support node (GGSN)
which contains the subscriber's session information when the subscriber has an
active session. When a mobile wants to use GPRS, it must first attach and then activate
a PDP context. This allocates a PDP context data structure in the SGSN that
the subscriber is currently visiting and the GGSN serving the subscriber's
access point. The data recorded includes
a. Subscriber's
IP address.
b. Subscriber's
IMSI.
c. Subscriber's
-Tunnel
Endpoint ID (TEID) at the GGSN
-Tunnel
Endpoint ID (TEID) at the SGSN
The Tunnel Endpoint ID (TEID)
is a number allocated by the GSN which identifies the tunnelled data related to
a particular PDP context.
Several PDP contexts may use
the same IP address. The Secondary PDP Context Activation procedure may be used
to activate a PDP context while reusing the PDP address and other PDP context
information from an already active PDP context, but with a different QoS
profile. Note that the procedure is called secondary, not the resulting PDP
contexts that have no such relationship with the one the PDP address of which
they reused.
A total of 11 PDP contexts
(with any combination of primary and secondary) can co-exist. NSAPI are used to
differentiate the different PDP context.
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